The San Juan Mountains of southwestern Colorado are part of an extensive Tertiary volcanic field consisting of a thick sequence of lavas, breccias, and ash-flow tuffs. These Tertiary volcanics are underlain by Precambrian, Paleozoic, and Mesozoic rocks which have been exposed in some areas as a result of uplift and erosion. The Needle Mountains and most of the western third of the San Juan massif consist of these older basement rocks.
The San Juan volcanics are the product of 3 episodes of volcanism that began about 40 million years ago and which lasted off and on for some 20 million years. The first series of eruptions produced massive amounts of andesite, rhyodacite, and mafic-rich quartz latite. These great outpourings of intermediate lava are preserved today in the famous San Juan Formation. The second pulse of volcanism produced vast quantities of silicic ash-flow tuffs and related breccias. Significantly, this episode of volcanism was responsible for most of the mineralization in the San Juans. Fifteen calderas have been recognized in the San Juans that date from this event. Significant mineralization is associated with many of these calderas. The third and final pulse of volcanism produced a new series of lavas consisting of basalts and high-silica alkali rhyolites. This bimodal sequence of basalt and rhyolite is radically different from the previous lavas. Shortly thereafter, volcanic activity in the San Juans subsided.
The precipitous Needle Mountains rise up in the very heart of the San Juan massif. They consist of an uplifted block of Precambrian igneous and metamorphic rocks exposed by deep erosion. This uplifted wedge of Precambrian crust consists of a belt of ancient metamorphic country rock intruded by a number of granitic plutons. The oldest intrusive rock in the Needle Mountains is the 1.78 billion year old Twilight Granite. This ancient foliated granite (or granite gneiss) comprises the bulk of the West Needle Mountains. Emplacement of the Twilight Granite was followed by a period of active tectonism in which a north-northeast trending foliation was imparted upon both the Twilight Granite and the surrounding country rocks (including some ancient Archean schists and gneisses and the Irving Greenstone). The younger Tenmile and Bakers Bridge granites were emplaced shortly thereafter followed by a hiatus of some 360 million years. Igneous activity then resumed with the emplacement of a number of large granitic plutons. The Eolus Granite, a biotite and hornblende-bearing monzogranite, was emplaced during this period of time.Gold, silver, lead, zinc, and copper mineralization occurs in the San Juans in the form of veins and fracture fillings, replacement bodies, and pipe or "chimney" deposits. The majority of these ore deposits are associated with Tertiary volcanics. In the Needle Mountains District, the ore bodies consist of gold and silver-bearing sulfides emplaced within faults and fractures in the ancient Precambrian country rock. The gold-bearing sulfides consist mostly of pyrite, sphalerite, galena, chalcopyrite, and tetrahedrite. These are typical hypabyssal polymetallic veins in that they are hosted in Precambrian crystalline basement rock. These types of deposits are rare in younger Paleozoic and Mesozoic strata and virtually unknown in Tertiary rocks. The heyday of the Needle Mountains District was the late 1870's when thousands of ounces of gold were produced. Unfortunately, the mines were quickly depleted of good-paying ore and the district foundered.
The remote and trackless West Needle Mountains provide the modern-day prospector with fertile but extremely challenging ground. Nestled between Lime Creek and the Animas River, the West Needle Mountains are not easily approached. No roads penetrate the range. Only one Forest Service trail is marked on the map; it meanders southward from Andrews Lake all the way to Crater Lake. Crater Lake is located at the head of Crater Creek, just northeast of Twilight Peak. To the enterprising and determined prospector, the ruggedness and inaccessibility of the West Needle Mountains makes them extremely attractive.
A number of factors combine to make the West Needle Mountains a prospector's dream. First of all, the area has never been thoroughly prospected. Secondly, the mountains are home to two well-documented lost mines - Levi Carson's rich gold-bearing vein on the north side of Twilight Peak and Tom Estes' sylvanite vein on the south side. Thirdly, the geology is favorable for the emplacement of small ore bodies. Sylvanite occurs in other mining districts in the San Juans including two fairly close ones - the Beartown District, located 16 miles northeast of the West Needle Mountains and the La Plata District, located 19 miles southwest. In the Beartown District, the sylvanite veins are emplaced within ancient Precambrian rocks. In the La Plata District, the sylvanite veins are emplaced within Permian and Jurassic sandstones. It is entirely conceivable that similar sylvanite veins exist somewhere along the southern flanks of Twilight Peak.
A straight line drawn from the La Plata sylvanite district northeastward to the Beartown District passes right through the southern flanks of the West Needle Mountains. This northeast-southwest trending line parallels the shear zones of a structural feature known as the Wichita Megashear Zone, which is superimposed upon the southwestern corner of Colorado. Could this linear relationship between the three locations be a function of the Wichita shearing event? Could this be the link between the sylvanite deposits of Beartown, the La Platas, and the Estes Mine?
Prospectors may want to concentrate on the southern portion of the West Needle Mountains, especially the southern flanks of Twilight Peak. A metal-detector may be useful in the search for sylvanite-bearing float. Prospectors must realize that the area is extremely difficult to penetrate and explore. In addition, the area of interest encompasses nearly 15 square miles of heavily forested mountain slopes.